1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image with a two-component developer. In addition, the present invention relates to an image forming method, an image forming apparatus, and a process cartridge using the developing device.
2. Description of the Related Art
Electrophotographic image forming methods used for dry image forming apparatuses such as laser printers, copiers and facsimiles typically include the following processes:
(1) charging the surface of an image bearing member such as a photoreceptor (charging process);
(2) irradiating the charged image bearing member with light so that the charges of the irradiated portions decay, thereby forming an electrostatic latent image on the image bearing member (irradiating process);
(3) developing the electrostatic latent image with a developer including a charged dry toner to form a visible toner image on the image bearing member (developing process);
(4) transferring the toner image to a recording material such as a paper sheet (transferring process);
(5) fixing the toner image to the recording material upon application of heat and/or pressure (fixing process); and
(6) cleaning the surface of the image bearing member so that the image bearing member is ready for the next image forming operation.
Recently, there is an increasing need for an image forming apparatus capable of performing high speed image formation while saving fixing energy. Therefore, toner capable of melting at a relatively low temperature is needed. Since a low temperature fixable toner has such a property as to be easily melted, the low temperature fixable toner is preferably used as a transparent toner because a glossy image can be formed with low fixing energy. However, when a low temperature fixable toner is prepared merely by decreasing the melting point of the toner, the toner tends to cause a problem in that the preservability of the toner deteriorates. In addition, when the toner is used for a two-component developer, the toner tends to cause a spent toner problem in that the toner adheres to the surface of the carrier when the developer is agitated in a developing device, thereby deteriorating the charging ability of the carrier.
Further, recent image forming apparatuses are required to produce high quality images, and when a pictorial image is formed, a technique in that high glossiness is imparted to the surface of a recording material is used to produce a clear glossy image.
In order to impart high glossiness to the surface of a recording material, a technique in that a transparent toner is applied to a non-image area of a color image on a recording material to decrease the difference in glossiness between the color image area and the non-image area; a technique in that a transparent toner is applied to the entire surface of a recording material; and the like, have been proposed. In addition, a technique in that a color toner image and a transparent toner image are formed on a recording material, and the images are heated by a fixing device, followed by cooling and peeling from the fixing device to prepare a glossy image is proposed. Using these techniques make it possible to produce copies having little difference in glossiness between an image area and a non-image area.
By contrast, in the printing field, treatments such as UV varnish printing, varnishing, and polypropylene film laminating are performed to control the glossiness of a desired portion of a printed recording material. For example, a technique in that after performing a usual printing operation, an additional spot printing operation is performed on a desired portion of the print using an additionally prepared plate and a UV varnish or the like to impart high glossiness to the portion is used. By using this technique, a print in which the portion subjected to the spot printing operation has as high glossiness as photographs and other portions thereof have relatively low glossiness can be produced. Namely, the print has large glossiness difference, and therefore the print can be differentiated from normal prints.
However, when such a print is produced using an offset printing method, it is necessary to prepare an additional plate for forming such a glossy portion. In addition, this method cannot be used for producing a small number of prints due to increase of running costs, i.e., the method can be used only for producing a large number of prints. Since electrophotography can perform image formation without using a plate, it becomes possible to produce such prints even when the number of the prints is small.
In attempting to produce images having different glossiness using electrophotography, a method in which a color toner image is formed on a recording material using at least one color toner (such as yellow, magenta or cyan toner) and a transparent toner, wherein an image portion having the transparent toner image has glossiness different from the glossiness of the color image portion by ±20% or more due to difference of the melting points of the color toner and the transparent toner; a method in which after a fixed color toner image is formed, an image is formed using a transparent toner while decreasing the fixing temperature to prepare an image portion having high glossiness and another image portion having relatively low glossiness; and a method in which initially a fixed glossy image is formed and then a non-glossy image is formed, followed by fixing, have been proposed. By using these methods, a copy having portions with different glossiness can be produced, but the glossiness of a glossy image portion of the copy is lower than the glossy portion of a pictorial print formed by the above-mentioned spot printing method.
One of background developing devices using a two-component developer including a toner and a magnetic carrier is illustrated in FIG. 1. Referring to FIG. 1, a background developing device 4a has two separated developer passages, i.e., a first developer passage (i.e., a developer supplying passage) for supplying a developer to a developing roller 5 (serving as a developer bearing member) and a second developer passage 10 (i.e., a developer agitating passage) for agitating the developer. The developer in the first developer passage is fed in a direction opposite to the feeding direction of the developer in the second developer passage 10 so that the developer is circulated in the two developer passages. In FIG. 1, numerals 401, 403 and 11 denote a first auger for feeding the developer in the first developer passage, a partition, and a second auger for agitating the developer in the second developer passage.
In the background developing device illustrated in FIG. 1, the first developer passage for supplying the developer to the developing roller 5 also serves as a developer collecting passage for collecting the developer passing through a development region so as to be used for developing electrostatic latent images on an image bearing member. Therefore, the concentration of toner in the developer decreases in the developer feeding direction in the first developer passage. Namely, the developer on the downstream side of the first developer passage relative to the developer feeding direction has lower toner concentration than the developer on the upstream side. Therefore, a problem in that images having uneven image density are formed is caused.
In attempting to avoid such an uneven density image problem, there are proposals for developing devices in which a developer supplying auger and a developer collecting auger for collecting the developer, which has been used for development, are arranged in different developer passages. Hereinafter, each of the background developing devices will be explained in detail.
One of the background developing devices is illustrated in FIG. 2. Referring to FIG. 2, another background developing device 4b includes a developer supplying passage 9 for supplying a developer to the developing roller 5, and a developer collecting passage 7 for collecting the developer passing through a development region at which the developing roller 5 is opposed to an electrostatic latent image bearing member 1, wherein the developer collecting passage 7 is separated from the developer supplying passage 9. Since the developer passing through the development region is fed to the developer collecting passage 7, the developer is not mixed with the developer in the developer supplying passage 9. Therefore, the toner concentration of the developer in the developer supplying passage 9 (i.e., the toner concentration of the developer fed to the developing roller 5) hardly changes.
However, the collected developer fed to the developer collecting passage 7 is supplied to the developer supplying passage 9 shortly after the developer is collected and a fresh toner is supplied to the collected developer (this developer is hereinafter sometimes referred to as a recovered developer). Therefore, even when the recovered developer has a proper toner concentration, problems in that uneven density images or low density images are produced occur. This is because the recovered developer (i.e., the mixture of the collected developer and the fresh toner) is not sufficiently agitated. The problems are remarkably caused when the developer has been used for developing images having a high image area proportion and the collected developer has a relatively low toner concentration. In FIG. 2, numerals 8, 6 and 209 denote first, second and third augers, and numerals 15 and 16 denote a center of the developing roller 5 and a developer thickness controlling member for controlling the thickness of the developer on the developing roller 5.
Another background developing device is illustrated in FIG. 3. In a background developing device 4c illustrated in FIG. 3, a developer supplying passage 9 for supplying a developer to a developing roller 5 is separated from a developer collecting passage 7 for collecting the developer passing through the development region at which the developing roller 5 is opposed to an electrostatic latent image bearing member 1. The developing device 4c further includes a developer agitating passage 10, which receives the developer, which has been fed to the downmost stream side of the developer supplying passage 9, and the collected developer, which has been fed to the downmost stream side of the developer collecting passage 7, to agitate the developers while feeding the mixed developer in the direction opposite to the developer feeding direction in the developer supplying passage 9.
In the developing device 4c, the developer used for development is fed to the developer collecting passage 7, and therefore the collected developer is not mixed with the developer in the developer supplying passage 9. Therefore, the toner concentration of the developer in the developer supplying passage 9 (i.e., the toner concentration of the developer fed to the developing roller 5) hardly changes.
In the developing device 4c, the collected developer is mixed with the developer fed through the developer supplying passage 9 without being used for development, and the mixed developer is agitated in the developer agitating passage 10. The mixed developer is then supplied to the developer supplying passage 9. Therefore, the above-mentioned problems in that uneven density images or low density images are produced are hardly caused. In FIG. 3, numerals 404, 405 and 27 denote a partition, another partition and a toner sensor for detecting the concentration of toner in the developer in the developer agitating passage 10.
The above-mentioned proposals have been made for a development operation using a color toner because it is described in the proposals that the purpose thereof is to stabilize the image density, and are not made for a development operation which uses a transparent toner to produce a glossy image portion while stabilizing the glossiness of the image.
For these reasons, the present inventors recognized that there is a need for an image forming method by which highly glossy images can be stably produced using a developer including a color toner and a carrier and another developer including a low temperature fixable transparent toner and a carrier without causing the hot offset problem and the high temperature preservability problem.